Refrigeration units operate according to well known principles. Items that a user wishes to keep colder than ambient air temperature are located in a refrigeration volume (referred to herein as the “box”) through one or more doors or other openings. Previously compressed and condensed refrigerant is expanded to create a temperature drop in an evaporator, through which air from the box is blown by one or more evaporator fans. The air from the box transfers heat into the refrigerant through the evaporator, and the cooled air is returned to the box. In the box, the cooled air absorbs heat from the box contents and the remaining box air. This is referred to herein as the cooling phase of the box. One or more thermostats measures the temperature in the box, and upon the measured box temperature reaching the required temperature (a low temperature set point), the compressor is turned off, ending the cooling phase.
Although modern insulation has decreased heat gain in the box of refrigeration units, intrusion heat still penetrates the box's insulated walls and doors. Heat also enters the box when doors are opened, particularly when the doors remain open for a long period of time. Regardless of the source of heat, the temperature of the air inside the box will gradually rise after the end of the cooling phase. This period is referred to as the heating phase of the box.
If the evaporator fans are stopped during the heating phase, the heated air naturally rises by convection and collects at the ceiling so that the temperature at the ceiling is greater than the temperature lower in the box. The air on the floor and at the mid level of the box will increase uniformly to a high temperature set point (incorporating some hysteresis relative to the low temperature set point to prevent excessive switching—although typically only a single temperature set point is actually set by the user) to start the cooling phase.
To avoid development of what is often considered to be an undesirable thermal gradient in the box and allow for more accurate control, most refrigeration units will continue to operate the evaporator fan(s) during the heating phase. The air thus circulated is passed through the evaporator where it entrains moisture collected in the condensate pan during the previous cooling phase. Therefore, although the humidity is reduced during the cooling phase, some humidity is returned during the heating phase only to be removed again in the following cooling phase. Thus, operating the refrigeration unit in this way results in additional energy consumption for running the evaporator fans and subsequent removal of humidity returned to the box air.